1. Field of the Invention
This invention relates to a process for preparing hydrocarbons, in particular, to a process for preparing hydrocarbons by hydrogenation of carbon dioxide over Fe-K/Al.sub.2 O.sub.3 catalyst, which is reduced under hydrogen atmosphere after activated in mixture of carbon dioxide and hydrogen.
2. Description of Related Art
Carbon dioxide is the so called `greenhouse gas`. Accumulation of carbon dioxide in the atmosphere caused by a huge amount of the fossil fuel consumption is now regarded as one of the major reasons for Global Warming.
To conserve the environment, it will be necessary to restrict the use of fossil fuel. However, it is expected that it will be necessary to use fuel and materials containing carbon atom continuously. A possible contribution to reducing carbon dioxide accumulation would be the chemical transformation of carbon dioxide into valuable compounds or fuel using suitable catalysts. This would provide a means of recycling carbon dioxide exhausted from combustion of fuel.
The material manufactured from the reduction of carbon dioxide should be consumed in a wide scope because carbon dioxide is exhauted in enormous volume. Therefore, the effective conversion of carbon dioxide to hydrocarbons would be regarded as one of the most promising routes for carbon dioxide fixation by catalytic process.
Processes for preparing hydrocarbons by hydrogenation of carbon monoxide have been studied widely and applied in commercial quantity. Catalysts and technology used for the hydrogenation of carbon monoxide also can be applied to hydrogenation of carbon dioxide. But chemical properties of carbon monoxide and carbon dioxide are quite different from each other, so the yield of hydrocarbons, especially hydrocarbons having 2 or more carbon atoms (hereinafter called "C.sub.2+ hydrocarbons") is extremely low.
It requires emergently new catalyst for preparing hydrocarbons in high yield without forming of by-product such as carbon monoxide. EQU CO.sub.2 .fwdarw.Hydrocarbons
There are two general methods for preparing C.sub.2+ hydrocarbons using carbon dioxide. The first method is comprised of two- step reactions, that is, conversion of carbon dioxide to methanol and continuous conversion of the methanol to hydrocarbons. The second method is reacting carbon dioxide with hydrogen to obtain hydrocarbons directly.
Conventional methods for preparing C.sub.2+ hydrocarbons using carbon dioxide according to the above first method are as follows.
Japanese patent application No. 89-190,638 discloses a method in which carbon dioxide is reduced at 300.degree. C., 10 atm over catalysts in a fixed bed reactor . The catalysts used in the above reduction are composed of CuO--ZnO--Al.sub.2 O.sub.3 and dealuminated H--Y Zeolite of SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio .ltoreq.10. As the result of reduction, the yield of hydrocarbon was 9.7% and the conversion of carbon dioxide was 20.3%.
Japanese patent application No. 92-120,191 discloses a method in which preparing of methanol from carbon dioxide, and C.sub.2+ hydrocarbons from methanol are performed in two reactors. Carbon dioxide is converted to methanol in the first reactor, and a mixture containing methanol is converted to C.sub.2+ hydrocarbons in second reactor. EQU CO.sub.2 +3H.sub.2 .fwdarw.CH.sub.3 OH+H.sub.2 O EQU CH.sub.3 OH.fwdarw.Hydrocarbons
The above two reactions are performed under different reaction conditions such as catalyst, temperature, pressure, etc. due to the difference of their optimum conditions at each reaction. The first reaction is performed appropriately at 250.degree. C., 80 atm, 4700 h.sup.-1 of the space velocity over CuO--ZnO--Cr.sub.2 O.sub.3 --Al.sub.2 O.sub.3 catalyst. On the other hand the second reaction is performed appropriately at 300.degree. C. , 1 atm, 1680 h.sup.-1 of the space velocity. As the result of the first reaction, the conversion of hydrocarbon was 32.1%, the selectivities to methanol and carbon monoxide were 24.9% and 7.2% respectively. The mixture passed through the first reactor was introduced into the second reactor. In the second reactor the conversion of carbon dioxide to hydrocarbon was not seen, but 32.1% of the methanol formed in the first reactor was converted to hydrocarbons.
According to the results of conventional studies, when C.sub.2+ hydrocarbons was prepared from carbon dioxide, the conversion of carbon dioxide was less than 35% and the yield of hydrocarbon was less than 30%.
G. A. Somoijai et al. reported that carbon dioxide was reduced by hydrogen at 300.degree. C., 6 atm over iron based catalyst to form hydrocarbons as desired products. As the result, 97% of hydrocarbons are methane and C.sub.2+ hydrocarbons formation is very low J. Catal., 52,291(1978)!.
Also C. H. Bartholomew et al. reported that carbon dioxide was reduced at 450.about.630 K, 1.about.11 atm in the molar feed ratio of H.sub.2 /CO.sub.2 =4 over catalysts. The catalysts were prepared by impregnation of silica with transition metal such as Co, Fe, Ru, etc. in 15% to weight of carrier used. As the result, methane was formed as major, C.sub.2+ hydrocarbons were less than 10% of hydrocarbons and the conversion of carbon dioxide was less than 15% J. Catal.87, 352(1984)!.
Also M. D. Lee et al. reported that carbon dioxide was reduced at 320.degree. C. , 10 atm, 600 ml/g/h of the space velocity and H.sub.2 /CO.sub.2 =4 over K-Fe(K=3 atom %) catalyst. As the result, the conversion of carbon dioxide was 35% and the yields of hydrocarbons and C.sub.2+ hydrocarbons s were 28% and 23%, respectivelyBull. Chem. Soc. Jpn, 62, 2756(1989)!.
The inventors of this invention have investigated a new method for preparing hydrocarbons, especially C.sub.2+ hydrocarbons by hydrogenation of carbon dioxide. As the result, this invention is completed by following method ; Fe-K/Al.sub.2 O.sub.3 catalyst was reduced with hydrogen and activated with the mixture of carbon dioxide and hydrogen, and then hydrogen and carbon dioxide (H.sub.2 /CO.sub.2 =1.0.about.5.0) were introduced to contact with the pretreated catalyst at 200.about.500.degree. C. , 1.about.100 atm, 500.about.20,000 h.sup.-1 of the space velocity.
In the process according to this invention, the conversion of carbon dioxide was raised much more than 50%, the yields of hydrocarbons and C.sub.2+ hydrocarbons are more than 50% and 45% respectively, and carbon monoxide generation is minimized.